Metal oxide films are widely used in the electronics industry. Preparation of such metal oxide films has been accomplished by physical vapor deposition techniques such as sputtering, electron beam (e-beam) evaporation, thermal evaporation, molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), by chemical vapor deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), and metalorganic chemical vapor deposition (MOCVD), and by sol-gel techniques and other chemical solution deposition techniques. Chemical solution deposition techniques have been generally viewed as less capital intensive (see, Lange, “Chemical Solution Routes to Single-Crystal Thin Films”, Science, vol. 273, pp. 903-909, 1996 and Schwartz, “Chemical Solution Deposition of Perovskite Thin Films”, Chemical Materials, vol. 9, pp. 2325-2340, 1997). Also, chemical solution techniques are not generally limited to flat surfaces.
Sol-gel techniques are not desirable for many industrial production processes. The sol-gel process uses the high reactivity of organometallic precursors and hydrolyzes these organometallic compounds to make various oligomers. These metal oxo oligomers have suitable viscosity to allow spinning into thin films, which can be fired into ceramic materials at high temperatures. The complication in such a sol-gel process is the uncontrollable polymerization of the metal oxo oligomers because of complex reactive species in the precursor solution. Therefore, the reproducibility of sol-gel processes can be poor which hinders the applications in industrial processes despite the low costs.
The use of organometallic compounds in chemical solution deposition techniques can be a drawback. As metal salts are typically insoluble in organic solvents, organic moieties have been added to the metal complexes in order to make such metal compounds soluble. This is often undesirable as new reactions and techniques must be developed to incorporate such soluble organic groups onto metal ions. In addition, the resulting organometallic compounds are usually difficult to handle because of their relatively higher reactivity than metal salts.
One problem with the processing of metal salts into ceramic thin films involves the hydrolytic properties of metal ions. Transition metal ions, such as titanium, niobium and tantalum, react with water violently to form metal oxides or metal hydroxides and precipitate out of solution during processing. A more desired methodology to achieve metal oxide films would be a chemical solution having the following properties: clean decomposition to pure ceramics; stable chemical solutions (no gelling) without any reactions before the firing stage; and, the desired viscosity for spin coating, spray coating, or film casting.
U.S. Pat. No. 6,589,457 by Li et al. is directed to deposition of metal oxides from aqueous solutions of water-soluble metal precursors and water-soluble polymers. While none of the examples included a polymer other than polyvinyl alcohol, Li et al. illustrate the continuing efforts in the development of chemical solution deposition processes for production of metal oxide films. The present invention is a continuation of those efforts.
An object of the present invention is to provide a chemical solution deposition method of forming metal oxide films, such a chemical solution deposition method including the deposition of a metal precursor and a soluble polymer where the polymer has binding properties for the metal precursor.
Another object of the present invention is to provide a chemical solution deposition method of forming high purity metal oxide films or epitaxial metal oxide films.
Yet another object of the present invention is to provide a metal oxide precursor solution having a long shelf-life time in comparison to typical sol-gel solutions.